1. Field of the Invention
The present invention relates generally to an optical signal source for a fiber optic interferometric sensor, and specifically to a broadband optical signal source for a fiber optic gyroscope. The present invention provides an apparatus for monitoring and correcting scale factor shifts in the light coming from the broadband fiber source.
2. Description of Related Art
Many fiber optic gyroscopes use a broadband fiber source to provide the light that is introduced into a fiber sensing coil for detecting rotation of the gyros. The typical broadband fiber source used in fiber optic gyros is a reverse pump, single-pass broadband fiber source 100. Such a configuration is shown in FIG. 1. This configuration uses a pump light source, such as a pump laser diode, that emits light at a given wavelength which is directed through a length of erbium doped fiber (EDF). The EDF has a core that has been doped with one or more of the rare earth family of elements, such as erbium. The light from the pump light source is introduced as an excitation signal into the EDF, which in turn causes the fiber to emit a light characteristic of the dopant. When an erbium doped fiber is supplied with a source of energy being pumped into the fiber, such as for example a wavelength of 1480 nm generated by the pump laser diode, the electrons in the erbium absorb the energy and jump to a higher energy state. This energy may later be released as light emitted from the EDF. When erbium is pumped with a laser at the appropriate wavelength, it emits a light having a wavelength between approximately 1525 to 1565 nanometers (nm). This light emitted from the broadband fiber source 100, which is broadband in nature, is then coupled into the fiber optic gyro 102. In the fiber optic gyro 102, the light passes from a fiber optic coupler 104 used as a multiplexer (MUX) through a multifunction integrated optics chip (MIOC) 106, which forms and processes counter-propagating waves used in fiber optic rotation sensor systems. The counter-propagating waves are then input into a fiber optic sensing coil 108. A phase shift between the counter-propagating waves develops as a result of the rotation of the fiber optic sensing coil 108. The counter-propagating waves are directed to a gyro photodetector 110, where the intensity of the light emitted from the sensing coil 108 is measured to determined the amount of rotation of the gyro. The light in the sensing coil 108 provides phase information which can be related to the gyro rotation rate through a term called scale factor. The scale factor is linearly related to the average wavelength of the light coming from the broadband fiber source 100.
Because of the broad spectral width of this light source, the scale factor becomes related to the weighted average of the spectrum, otherwise referred to as the centroid wavelength. It has been shown that when the erbium doped fiber (and to a lesser degree the other fiber optic components, such as the sensing coil 108) are exposed to harsh environments, changes in the broadband fiber source can cause a large shift in the centroid wavelength, resulting in a large scale factor error. For instance, exposure to ionizing radiation can adversely affect the centroid wavelength of light sources using the EDF. FIG. 2 illustrates the effects of ionizing radiation on the spectrum of the EDF light source over a continuum of levels of ionizing radiation. A typical spectrum for the broadband light source before exposure to ionizing radiation is represented by the graph at 0% radiation. Full exposure to a predetermined dose of ionizing radiation is represented by the graph at 100% radiation, where the remaining graphs show the spectrums of the broadband light source at doses of ionizing radiation which are fractional amounts of the 100% dose. As can be seen, the relative intensity of the EDF light source decreases with respect to the pre-exposure spectrum and the shape of the spectrum of the broadband light sources changes as the total dose of ionizing radiation increases. The 100% dose of ionizing radiation resulted in an approximately a 500 ppm shift in the centroid wavelength from the pre-exposure spectrum.
The spectrum from a broadband light source is made up of a composite of several emission peaks. As the radiation damages the EDF, the different emission peaks experience different levels of attenuation. This has the effect of shifting the centroid wavelength. The shift in the centroid wavelength can correspond to a scale factor shift over a 1000 parts per million. For fiber optic gyroscopes being used in applications requiring a high degree of accuracy, this creates an unacceptable level of error. There is a need for stabilization of the scale factor of a broadband fiber source used in fiber optic gyros when exposed to ionizing radiation and other external influences.
One solution for stabilizing the scale factor of a broadband fiber source is described in co-pending U.S. patent application Ser. No. 09/104,496 filed Jun. 25, 1998, and assigned to Litton Systems, Inc., which discloses using a broadband fiber filter to only allow a bandwidth of light narrower than the spectral width of the broadband light source to propagate through to the fiber optic gyro while attenuating all light outside of its operating bandwidth. This narrows the spectral width of the broadband fiber source which, in turn, reduces the centroid wavelength shift resulting when the EDF is exposed to ionizing radiation. Referring now to FIG. 3, a graphical illustration of the reduction in centroid wavelength shift for several bandpass filters is shown for the broadband fiber source of FIG. 2. As set forth above, a 500 ppm centroid wavelength shift occurred after exposing an EDF source made with HG-980 fiber to predetermined dose of ionizing radiation. After positioning a bandpass filter after the broadband fiber source, the centroid wavelength shift is reduced significantly, where lines 30, 32 and 34 of FIG. 3 show the respective reduced centroid wavelength shift for filters having a bandwidth of 12 nm, 10 nm, and 8 nm. Given the overall shape of the broadband fiber source spectrum, it can be seen that the best centroid wavelength shift reductions occur when the filter center wavelength is between 1558 to 1559 nm.
The best centroid wavelength shift reductions typically result from bandpass fiber filters having the smallest bandwidths. However, the intensity of the smaller bandwidth signals tend to be weaker than the pre-filtered broadband signals. This allows the quality of these weaker signals to be more easily degraded by noise and other undesirous factors. Thus, there is a need for an apparatus which provides scale factor stabilization of a broadband fiber source while maintaining the intensity of the light signal at a high enough intensity so as not to easily be degraded by noise. Moreover, for applications requiring extreme precision, a scale factor accuracy of a fraction of a part-per-million is required through adverse environments. Thus, there is a need for a system which provides scale factor stabilization by monitoring and correcting the centroid wavelength shift of the spectrum of the broadband fiber source when exposed to ionizing radiation.